Impact of Dust Radiative Forcing upon Climate - Mineral Dust: A Key Player in the Earth System - page 346

practices. These might be constrained with deposition inferred from geological
surveys. Only with both source extent and strength constrained can we quantify
the complete feedback loop between dust, climate, and vegetation.

13.5 Conclusion

Radiative forcing by dust aerosols changes surface temperature and rainfall along
with variables like wind speed and vegetation that feed back upon the dust concen-
tration. ESMs demonstrate that adjustment to the forcing occurs over the extent of
the perturbed circulation. As a result, the temperature perturbation at any location
at equilibrium is not directly related to the local temperature tendency associated
with the forcing. In regions of frequent vertical mixing by deep convection, the
anomalous surface temperature is controlled primarily by the aerosol forcing at
TOA rather than the surface. The strong coupling of surface air temperature to TOA
forcing is implicitly recognized in the classic definition of climate sensitivity that
is the ratio of these two variables (e.g., Forster et al. 2007 ). Outside of convecting
regions, the influence of surface forcing is similarly limited. Many models calculate
warming of the Sahara by dust, despite surface forcing of contrasting sign. The sign
of TOA forcing also varies inconsistently among these models, indicating that lateral
transports of energy need to be considered to interpret the temperature response.
Surface temperature anomalies depend not only upon changes by dust to the energy
budget but also changes to precipitation and humidity, as shown by the cooling
within the Sahel as the TOA forcing becomes increasingly positive (Fig. 13.5 ).
In general, transports of energy and moisture should be diagnosed along with the
forcing to attribute the climate response calculated by ESMs.
Models generally predict a reduction of global precipitation by dust radiative
forcing. This has been interpreted in the literature as a simple consequence of the
negative surface forcing beneath the aerosol layer that leads to a compensating
reduction in surface evaporation. However, while precipitation is reduced for all
three prescriptions of particle shortwave absorption in Table 13.1 , the reduction is
not proportional to dimming of the surface. The decrease of global precipitation
(and evaporation) is smallest for the most absorbing particles, even though the
magnitude of the surface forcing is largest. Xian ( 2008 ) demonstrates how the
influence of positive TOA forcing upon surface air temperature and humidity can
increase surface evaporation and precipitation over the ocean. While the precise
relation between global precipitation and forcing needs further investigation, the
more general point is that precipitation depends upon forcing at both TOA and
the surface (Ming et al. 2010 ).
Regional precipitation responds in a complicated way to dust radiative forcing.
Dust generally reduces oceanic precipitation downwind of the aerosol layer, beneath
which evaporation is reduced, as in the tropical Atlantic. Over the Sahel, however,
the sign of the precipitation anomaly seems to follow the local TOA forcing
(Table 13.2 ). Figure 13.7 shows that Sahel precipitation increases for positive TOA
forcing, despite shortwave heating within the dust layer and reduced radiation